This invention relates to an in-line combined bleaching and dewaxing process for treating edible vegetable oils to produce a vegetable oil that has acceptable storage characteristics.
Crude vegetable oils are extracted from plant tissue and include such varieties as corn, milo, rapeseed (canola), ricebran, sunflower and safflower.
Crude vegetable oils contain undesirable minor components or impurities such as pigments, free fatty acids, phospholipids and oxidation products, which can cause undesirable color and/or "off flavors" in the finished vegetable oil. In addition, certain higher melting components must be removed from the vegetable oils if they are to be used in food products such as salad oils and dressings which must be refrigerated. Unless removed, the higher melting constituents would crystallize and separate when the vegetable oils are stored at refrigeration temperatures.
The conversion of crude vegetable oils into an acceptable product may require several treatment steps including degumming, alkali refining, bleaching, winterization, dewaxing and deodorization.
The term "winterization" is applied to a process for removing high melting material from oils whereby the oils are carefully cooled to low temperatures for extended periods of time to permit precipitation of solid material. Solid material can then be removed by filtration or other separation procedures. Examples of winterization processes are disclosed in U.S. Pat. Nos. 2,200,982 to Dedlow, 3,048,491 to Gooding and 4,035,402 to Levine.
Alkali refining of a vegetable oil involves its treatment with an alkali, such as sodium hydroxide, to remove free fatty acids, phospholipids, trace metals, pigments and oxidation products. The alkali solution neutralizes the free fatty acids contained in the crude vegetable oil, producing a soap stock which can be continuously removed by centrifugation. Phospholipids, also referred to as phosphatides, are soluble in the anhydrous vegetable oil, but after treatment with an alkali solution precipitate out with the soap stock and can also be removed.
Other alkali solutions, such as sodium bicarbonate, calcium hydroxide, potassium hydroxide, magnesium hydroxide, ammonia, and some organic bases can also be used in alkali refining a crude vegetable oil. Examples of alkali refining treatments are disclosesd in U.S. Pat. No. 3,943,155 to Young.
An alternative to "chemical" alkali refining, is physical refining whereby oil impurities are removed by physical means in the degumming, bleaching, dewaxing and steam refining/deodorization steps. During degumming, crude vegetable oil is mixed with a small amount of water (1-3%), agitated to achieve hydration of gums, primarily phospholipiods, thus making them insoluble in the vegetable oil, and further the hydrated gums are separated from the oil by such means as centrifugation. When the degumming is done at ambient or lower temperatures, a partial removal of waxes can also be achieved.
Alkali refining and degumming are alternative approaches that are generally used as preliminary steps in the purification of crude vegetable oils. Either alkali refining or degumming is generally used in combination with subsequent bleaching, dewaxing and deodorization treatments of the vegetable oil.
The purpose of bleaching step is to further purify the vetgetable oil by removing residual phospholipids, trace metal complexes and pigments such as carotene, chlorophyll and related compounds, as well as oxidation products. Moreover, where the bleaching step is preceded by alkali refining, the bleaching treatment can also remove residual soaps left by the alkali refining treatment.
In a conventional bleaching process, the vegetable oil is mixed with a bleaching clay which serves as an adsorbent. The bleaching clay-vegetable oil mixture is then heated for a period of time, and filtered to separate the spent adsorbent from the decolorized oil. Ordinarily, much of the bleaching action occurs during the holding of the oil/clay mixture at elevated temperatures under vacuum with intense agitation.
In the situation where degumming is used prior to the bleaching step, the bleaching is generally conducted in the presence of phosphoric acid which reacts with residual phospholipids, as well as with the metals present in the vegetable oil converting the metals into phosphates.
Activated carbon can also be used in place of the bleaching clay as an an adsorbent, however, for economic reasons, if it is used at all, it is generally mixed with the bleaching clay.
The bleaching step can be conducted under atmospheric pressure, however, it is usually done under vacuum conditions to avoid oxidizing the bleached oil. Examples of bleaching treatments are disclosed in U.S. Pat. Nos. 3,673,228 to Harris, 3,943,155 to Young and 3,955,004 to Strauss et al.
The bleached vegetable oil still contains small amounts of high melting point components, such as saturated glycerides, wax esters, sterol esters and hydrocarbons which can crystallize and precipitate at ambient temperatures, and especially at refrigeration temperatures. It is these high melting point compounds, generally referred to as waxes, which are responsible for the haze and cloudiness of an oil.
The conventional dewaxing process includes slow chilling of the oil to temperatures sufficient to crystallize the waxy components from the crude oil, preferably under gentle agitation. The crystallized components are then generally removed by a cold filtration step. U.S. Pat. Nos. 3,943,115 to Young, 3,994,943 to Gibble and 4,035,402 to Levine disclose various processes for dewaxing vegetable oils. U.S. Pat. No. 2,625,482 to Mattil discloses a dewaxing process for lard.
Following the bleaching and dewaxing steps, the oil may be deodorized, usually with steam under vacuum at a high temperature. Steam deodorization involves the contacting of steam with free fatty acids and other volatile odorous and off-flavor materials often present in the vegetable oil which are responsible for the undesirable odor and taste of non-deodorized oil. U.S. Pat. No. 3,506,969 to Baker et al discloses a typical steam deodorization process.